Abstract

In this text one can see the process of the preliminary design to achieve a solution for a high
efficiency propelling system for the A boat competing in the Dong solar challenge. Specifically,
this document reports the research made to achieve a solution for the mechanical design of
this motor from some features given by the foregoing electric design. To achieve this goal, a
methodical design will be performed. During this methodical design, 2 different concepts are
considered. Almost every component in both concepts is the same, as they are the
components which accomplish the main functions of the motor, which is turning electric
power into movement. They differ in the bearing disposition and the shafts shape, which will
be an important point of discussion. The aim of this report is then choosing between the two
concepts according to some requirements, like the shape of the rotor determined by the
electric design, and some wishes, like high efficiency, low weight and being easy to attach to
the boat. The calculations are made to orientate the discussion about which concept is the
best, but they are not definitive to set the final dimensions of the different components.
Before being able to discuss which one is better, some calculations need to be made, as they
make the difference within the two motors.
The bearings’ life is considered a very important point of discussion. After calculating the
forces applied on the bearings using the finite element software LISA 7.7, it is found that in
both motors, there is at least one bearing which life is too short. For this reason, needle
bearings are adopted to solve the problem in the motor 1. For the motor 2, this option cannot
be adopted so this almost discards this concept as an option.
Then the calculations for the connection between the shaft and the rims through the pins are
made. The three possible failures of the connection considered are shearing forces on the pin,
shaft tear and rim tear. The lowest safety factor found is the one concerning the shearing
forces on the pin, so if this connection fails, it will be here. Nevertheless, as this safety factor is
big enough, the connection is validated.
After that, a first simulation to know the behaviour of the shafts is made. Both shafts are
simulated under the same conditions of performance with the Solidworks Simulation Express
extension. The two principal matter of discussion are the von Mises stress and the angular
displacement of the nodes. The result of the simulation shows that the shaft in the motor 1 is
better in both items of discussion, with a safety factor for the stress of 2,54 and a
displacement of 0,2 mm.
Other items considered during the design such as assembly, fabrication and weight don’t turn
to be determinant because there is not a big difference between them in the two different
concepts.
After discussing each requirement and wish, one can see that the motor 1 is better, or at least
equal, than the motor 2 in every item discussed. For this reason, the concept chosen for the
motor is the motor 1.